1. Field of the Invention
This invention relates generally to an apparatus for controlling vehicle creep in response to accelerator pedal and brake pedal signals using a powertrain that includes an electric motor and launch clutch.
2. Description of the Prior Art
In a conventional vehicle equipped with an internal-combustion and automatic transmission, the vehicle can creep forward or backward when the shift lever is in a forward or reverse driving position. When the driver's foot is off of the accelerator pedal, and either off or slightly depressing the brake pedal, the vehicle will creep forward or backward at low speed, e.g. 10 kph. In the conventional configuration, the engine shaft is always rotating at idle speed or higher. The transmission pump, driven by the engine shaft, provides hydraulic pressure to the transmission. The torque converter is allowed to slip and maintain vehicle speed at a constant velocity, i.e. the desired creep speed.
A vehicle equipped with modular hybrid transmission (MHT) creeps due to power supplied by the internal combustion engine or an electric motor, when the engine is not running. Since a MHT vehicle powertrain includes no torque converter, vehicle creep creates unique challenges for the vehicle controls. When the engine is on, the engine disconnect clutch is closed and the engine shaft rotates at idle speed or higher. Thus, the transmission input shaft drives the transmission pump, which provides pressure for operation of the transmission clutches. A launch clutch is allowed to slip and maintain vehicle speed constant, similar to function of a torque converter in a conventional vehicle.
When the engine is off, however, electrical energy is wasted when the electric motor and pump are used solely to provide hydraulic pressure to the transmission. An auxiliary pump can be used to maintain pressure, but this also uses electrical energy unnecessarily. An auxiliary pump would have to be quite large in order to create enough pressure when the input shaft is not rotating, thus adding cost to the vehicle.
Producing vehicle creep using the electric motor requires a method of ensuring that the electric motor is driving the transmission pump and providing hydraulic pressure to the transmission when needed for creep, and then allowing the motor to spin down and conserve energy when not needed.
A need exists in the industry for a method that controls vehicle creep with either engine drive or motor drive, and manages transitions between engine drive and motor drive.